The invention relates to inverters in general and more particularly to an inverter for induction heating. Such an inverter supplies high frequency alternating current to the induction heating coil which forms a tank circuit that has varying electrical characteristics according to the material and size of the workpiece and the temperature of the workpiece being heated. The load in the induction heating installation has an inductive component, a capacitive component and a resistive component. Consequently, the load is susceptible to frequency changes and presents impedance characteristics to the inverter that vary drastically. In recent years, there has been a substantial amount of work devoted to the development of a high power solid-state power supply for driving an induction heating load. Such devices generally convert direct current to an alternating current which flows through the load. One of the most common of these devices is a solid-state inverter having a constant current supplied from a D.C. source, which current is alternately switched through the load in different directions by two distinct sets of switching devices, generally silicon controlled rectifiers (SCR's). This type of solid-state device has been used in tandem with a power rectifier which converts available three phase alternating current into direct current. This direct current is then directed to the inverter which changes the direct current into a single phase alternating current of a controllable high frequency. Frequency of the inverter is controlled by the rate at which gating signals are provided to the SCR's. One form of such inverter is a current-source, parallel-compensated inverter, well known in the induction heating art.
A particularly advantageous use of such inverters is in sequential induction heating of workpieces in an assembly line fashion. Workpieces to be heat treated may be passed to an appropriately designed induction heating coil driven by a power inverter such as that previously described. For each workpiece to be heated, the inverter is energized to drive the heating coil to heat treat the workpiece in the desired manner.
A problem with such applications of power inverters that is particularly serious when used for varying loads is that the inverter is difficult to start commutating. It is usually impossible to start a power inverter by merely providing gating pulses to the SCR's of the inverter in a manner similar to a steady state condition. At start up, there is no energy in the load for commutating the SCR's. This starting problem is further compounded when the load is spaced from the inverter so that a substantial inductance is created by connecting leads between the inverter and the load. Because of these difficulties, a substantial amount of work has been devoted to providing an arrangement for starting the power inverter used in induction heating. An advantageous arrangement has been described in U.S. Pat. No. 4,511,956. In accordance with this reference, a power inverter is provided, including a starting inverter having a D.C. input and A.C. output, with a frequency controlled by a series of repeating gating commands. An arrangement for selectively applying the A.C. output of the starting inverter across the load is provided with means responsive to the existence of a given energization condition of the load to energize the main power inverter, and remove the auxiliary starting inverter from the load. The auxiliary starting inverter can be a separate and distinct electrical device having relatively small, inexpensive components in comparison with the power inverter and is intended to drive the load at near its resonant frequency at significantly lower output levels than the power inverter operational levels. Thus, the starting inverter may be used to develop a voltage across a load until the load was charged sufficiently to allow commutation of the power inverter SCR's. It has been found that while this device offered improvements over prior starting arrangements, there still remained a percentage of attempts to energize the system when the power inverter failed to start, i.e., commutation was not commenced. It will be appreciated that in serial heating applications, a failure to energize the power inverter will result in a workpiece not being heat treated.
It has been recognized that a power inverter of the type described has an inherent level of output voltage instability, below which level the power inverter is inconsistent at start-up, particularly when the load to be heated has a relatively low Q (below e.g., 6). If the starting inverter output level is below this level of instability, commutation failure may occur when there is an attempted start-up of the power inverter.